Ethylene manufacture entails the use of pyrolysis or cracking furnaces to manufacture ethylene from various gaseous and liquid petroleum feedstocks. Typical gaseous feedstocks include ethane, propane, butane and mixtures thereof. Typical liquid feedstocks include naphthas, kerosene, and atmospheric/vacuum gas oil. When gaseous or liquid hydrocarbon feedstocks are pyrolyzed in the presence of steam, significant quantities of ethylene and other useful unsaturated compounds are obtained. Steam is used to regulate the cracking reaction of saturated feedstocks to unsaturated products. The effluent products are quenched and fractionated in downstream columns, and then further reacted or processed depending on need.
Fouling of cracking furnace coils, transfer line exchangers (TLEs) and other heat transfer surfaces occurs because of coking and polymer deposition. The fouling problem is one of the major operational limitations experienced in running an ethylene plant. Depending on deposition rate, ethylene furnaces must be periodically shut down for cleaning. In addition to periodic cleaning, crash shutdowns are sometimes required because of dangerous increases in pressure or temperatures resulting from deposit buildup in the furnace coils and TLEs. Cleaning operations are carried out either mechanically or by passing steam and/or air through the coils to oxidize and burn off the coke buildup.
A major limitation of ethylene furnace run length is coke formation in the radiant section and transfer line exchangers (TLEs). The coke is normally removed by introducing steam and/or air to the unit which in effect burns off carbonaceous deposits. Since coke is a good thermal insulator, the furnace firing must be gradually increased to provide enough heat transfer to maintain the desired conversion level. Higher temperatures shorten the tube life, and tubes are quite expensive to replace. Additionally, coke formation decreases the effective cross-sectional area of the process gas, which increases the pressure drop across the furnace and TLEs. Not only is valuable production time lost during the decoking operation, but also the pressure buildup resulting from coke formation adversely affects ethylene yield. Run lengths for ethylene furnaces average from one week to four months depending in part upon the rate of fouling of the furnace coils and TLEs. This fouling rate is in turn dependent upon the nature of the feedstock as well as upon furnace design and operational parameters. In general, however, heavier feedstocks and higher cracking severity results in an increased rate of furnace and TLE fouling. A process or additive that could increase run length would lead to fewer days lost to decoking and lower maintenance costs.
Significant effort has been exerted over the past twenty years in developing phosphorus, in numerous forms, as a coke inhibitor. See U.S. Pat. Nos. 3,531,394 to Koszman (phosphoric acid); 4,105,540 to Weinland (phosphate and phosphite mono and diesters); 4,542,253 and 4,842,716 to Kaplan et al. (amine complexes of phosphate, phosphite, thiophosphate and thiophosphite mono and diesters); 4,835,332 to Kisalus (triphenyl phosphine); and 4,900,426 to Kisalus (triphenyl phosphine oxide). Compared with other element-based additives, many of these phosphorus-based antifoulants have performed extremely well with respect to coke suppression in both lab simulations and industrial applications; however, some have yielded detrimental side effects preventing prolonged usage in many situations, e.g., contributing to corrosion, impairing catalyst performance, or the like.
Convection section corrosion has been a problem with many phosphorus-based anticoking additives of the prior art. Along the path of the convection section tubing, conditions are constantly changing. Heated steam and hydrocarbon are typically introduced to the section separately and then mixed well before entering the radiant section. During the numerous passes that the streams experience, separated or mixed, there can be temperatures, pressures, and compositions which enhance the conversion of antifoulants to detrimental corrosive by-products. A product which is an excellent coke suppressant may also be an extremely corrosive species if it accumulates in the convection section.
Once additives pass through the convection, radiant, and TLE sections, they are subject to effluent quench conditions. In a very simplified view, heavy products concentrate in the primary fractionator, water quench tower, caustic tower and/or compressor knock-out drums, while the lighter components are collected in columns downstream of the compressors. Accumulation of coke inhibitors and their cracked by-products is dictated mainly by their physical properties. Briefly, inhibitor by-products with high boiling points are condensed early in the fractionation process while lighter ones progress to the later stages.
Accumulation of antifoulants and/or their by-products in the radiant and TLE coke, primary fractionator, or water quench tower, is for the most part acceptable. These sections process and collect many other heavy products which are quite impure and thus, trace amounts of an additive generally do not have a significant impact.
In contrast, additives and/or by-products that go past the caustic tower and compressor sections can be a significant problem. Past these sections, purity becomes an important issue since the downstream fractionation generally separates the unsaturated products into high purity chemicals. The presence of phosphorus-containing products which might adversely affect the performance of catalysts used to process these lighter components is unacceptable.
Many phosphorus-containing products are good ligands and can adversely affect the catalyst performance. The phosphorus by-product which is of greatest concern is phosphine (PH.sub.3). This by-product is extremely low-boiling (-88.degree. C.). In fact, it has basically the same boiling point as acetylene (-84.degree. C.), a hydrocarbon by-product which is often catalytically hydrogenated to the more desired ethylene.
Accordingly, there remains a need for a phosphorus-based anticoking additive for cracking furnaces which is essentially free from contributing to corrosion and from forming catalyst impairing by-products.